Blend olive oil, lemon juice, garlic, harissa, cumin, black pepper, and salt together in a blender or food processor until marinade is smooth. Place tuna in a bowl and pour marinade over tuna; toss to coat. Marinate in the refrigerator for at least 1 hour.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Thread 1 piece tuna, 1 piece red bell pepper, 1 piece yellow bell pepper, and 1 piece onion onto 1 skewer; repeat 4 times. Repeat with remaining ingredients using the remaining 5 skewers.
Cook skewers on grill until fish flakes easily with a fork, about 10 minutes.